The present invention relates to a high solid-type clear coating composition and a method of producing a coated article by utilizing such a clear coating composition, and more particularly to a clear coating composition capable of providing clear topcoats excellent in acid resistance, durability and appearance without suffering from wrinkling after baking, which is suitable for automobiles, buildings, outdoor constructions, etc., and a method of producing a coating by using such a clear coating composition.
Recently, increasingly more attention has been paid to the deterioration of coatings for automobiles, buildings, outdoor constructions, etc., by acidic rain. Topcoats for automobiles, etc. are conventionally produced using acrylic resin paints, polyester resin paints, etc., but when coatings produced from such paints are brought into contact with acidic rain, spots, discoloration, etc., are locally generated, so that the coatings are likely to be damaged or broken.
By using silicone resin paints, fluorine resin paints, polyurethane resin paints, etc., high resistance to acidic rain can be achieved. However, these resins are generally expensive, and their paint quality is are not sufficiently good. Thus, various attempts have been made of provide coatings with high resistance to acidic rain by adding isocyanates, etc., to paints based on acrylic resins, polyester resins, etc.
In the meantime, for the purpose of saving resources and preventing air pollution, so-called high solid-type paints containing amino resins and aqueous paints have recently been used for topcoats of automobile bodies, etc. As high solid-type clear paints for topcoats for automobiles, etc., those containing acrylic resins together with melamine resins are generally used for the purpose of increasing their scratch resistance.
In the process of producing a coating for automobiles, an electrodeposition coating and an intermediate coating are first produced, and then an undercoat (base coat) is formed by using a base paint (undercoat paint containing coloring agents such as metallic pigments, etc.), and thereafter a clear paint is applied to this undercoat by a wet-on-wet system using a high solid-type clear paint, and the two coating layers (undercoat and topcoat) are simultaneously baked by heating. This system is called a "2 coat-1 bake" system.
However, when a high solid-type clear paint containing an acrylic resin and a melamine resin is applied to an aqueous undercoat and baked, the resulting coating is likely to suffer from damage such as wrinkling, etc., so that its appearance is not satisfactory. Also, cross-linking is insufficient in the coating layer, leading to poor hardness, weathering resistance, etc.
Under such circumstances, active development has been made to provide clear paints which can provide, as paints for automobile bodies, coatings having well-balanced properties such as good resistance to acidic rain, good appearance, good weathering resistance, etc., by a low-pollution system (as a high solid-type using little or substantially no solvents), on aqueous undercoats.
For instance, Japanese Patent Laid-Open No. 1-158079 discloses a coating composition comprising a particular acrylic copolymer, an alkyl ether-substituted melamine resin, and optionally a blocked polyisocyanate.
Also, Japanese Patent Laid-Open No. 1-310773 discloses a method of producing a clear coating comprising the steps of applying a heat-curable aqueous paint containing pigments to a surface to be coated, drying or curing it, and applying a clear paint containing a polyisocyanate compound as a cross-linking agent to the surface, the molecular weights of the resin portions between cross-linked points in the cured clear coating being adjusted to 200 or less.
However, although coatings produced by the above composition and the method have certain levels of acid resistance and scratch resistance, they fail to show good surface appearance together with good acid resistance and durability, which are required on clear paints applied onto aqueous undercoats.